Core/shell polymers are well-known; such polymers typically have a hydrophilic portion and a hydrophobic portion comprising a latex particle morphology consisting of an inner "core", surrounded by an outer "shell". Core/shell polymers are commonly used to disperse molecules or particles, such as pigments, which are ordinarily insoluble in water, but which, after association with the core/shell polymer, form stable dispersions in water. Encapsulation occurs when the hydrophobic portion of the polymer associates with the water-insoluble molecule, and the hydrophilic portion of the polymer disperses with water.
U.S. Pat. No. 4,597,794 discloses the dispersion of pigments in an aqueous vehicle, using aqueous binders comprising both hydrophilic and hydrophobic components. The dispersion of the pigment is followed by centrifugation to eliminate the non-dispersed components such as agglomerates. Examples of the hydrophilic component comprise polymers of monomers having a mainly additively polymerizable vinyl group, into which hydrophilic construction portions such as carboxylic acid groups, sulfonic acid groups, sulfate groups, etc. are introduced by using a predetermined amount of an .alpha.,.beta.-unsaturated monomer such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, itaconic acid monoester, maleic acid, maleic acid monoester, fumaric acid, fumaric acid monoester, vinyl sulfonic acid, sulfoethyl methacrylate, sulfopropyl methacrylate, sulfonated vinylnaphthalene, etc. Examples of the hydrophobic portion comprise polymers of monomers selected from the group consisting of styrene, styrene derivatives, vinylnaphthalene, vinylnaphthalene derivatives, and .alpha.,.beta.-ethylenic unsaturated carboxylate of aliphatic alcohol having C.sub.8 -C.sub.18. In addition to the foregoing monomers, also included are acrylonitrile, vinylidene chloride, .alpha.,.beta.-ethylenic unsaturated carboxylate, vinyl acetate, vinyl chloride, acrylamide, methacrylamide, hydroxyethyl methacrylate, hydroxypropyl methacrylate, glycidyl methacrylate, N-methylol acrylamide, N-butoxymethyl acrylamide, etc.
U.S. Pat. No. 5,082,757 discloses encapsulated toner compositions comprising a core and a hydroxylated polyurethane microcapsule shell derived from the polycondensation of a polyisocyanate and a water soluble carbohydrate. The core comprises a polymer binder, pigment, dye, or mixtures thereof. Examples of the polymer binder include polymerized monomers selected from the group consisting of acrylates, methacrylates, and olefins including styrene and its derivatives.
U.S. Pat. No. 5,461,125 discloses waterborne core-shell latex polymers useful as adhesive films, rather than super-dispersion stability. The core comprises a (co)polymer comprising a (meth)acrylate ester, while the shell comprises a copolymer, the copolymer comprising (1) a nitrogen-bearing ethylenically-unsaturated free-radically polymerizable monomer, (2) at least one (meth)acrylate ester of about a C.sub.1 to C.sub.14 alcohol, and (3) an optional ethylenically-unsaturated free-radically polymerizable silane monomer, wherein the nitrogen-bearing ethylenically-unsaturated free-radically polymerizable monomer comprises about 15 to 60 wt % of the shell and wherein the core comprises about 40 to 85 wt % of the weight of the total core-shell latex particle. The polymers obtained by practicing the teachings of the disclosure have molecular weights of 400,000 or more, and the total low T.sub.g component (less than -10.degree. C.), where T.sub.g is the glass transition temperature, is greater than 60 wt %.
U.S. Pat. No. 5,656,071 discloses ink compositions useful for ink-jet applications. These compositions include an insoluble pigment and a polymeric dispersant. In one embodiment, the polymeric dispersant comprises block or graft copolymers comprising a hydrophilic polymeric segment (particularly an acrylate or methacrylate copolymer) and a hydrophobic polymeric segment which includes a hydrolytically stable siloxyl substituent.
Heretofore, ink-jet printers have not had printing performance and durable print properties of competitive printer technologies. The foregoing cited patents do not provide for useful, durable film-forming properties upon removal of the water or solvent. Film durability includes wet and dry rub resistance, highlighter smear-fastness, light-fastness, and waterfastness (e.g., hot and cold water, under spill, soak, and rub conditions).
In particular, formulating an ink-jet ink often involves compromising competing interests. For example, it is possible to enhance one property, such as durable film-forming of the colorant. However, such enhancement usually results in the degradation of another property, such as printing stability associated in thermal ink-jet with resistor fouling or nozzle clogging (kogation or decap--nozzle crusting).
In the use of core/shell polymers to enhance water-dispersibility of water-insoluble or water-non-dispersible species, such as colorants (pigments, water-insoluble dyes, metal particles, and the like), the issue of competing interests also arises. Here, the competing interests lies in adhesion between the colorant and the core/shell polymer versus the ability to form a durable layer of ink on the print medium upon drying. Durable polymers usually do not bond well to colorants, whereas polymers that bond well to colorants usually have poor solvent resistance. Thus, enhancing one of these properties has been found to result in a degradation of the other.
Thus, there is a need to provide an improved core/shell polymer for dispersing water-insoluble and water-non-dispersible colorants while enabling the formation of an ink capable of forming durable, smear-fast films of ink on the print medium upon drying.